Summary of Research on CO₂ Injection for Hydrogen Production and Carbon Sequestration

Injecting carbon dioxide (CO₂) underground could accelerate natural hydrogen production while turning the greenhouse gas into solid rock, but this approach is only effective under specific conditions, according to new research from Princeton University and Ohio State University. The study used geological modeling to investigate whether CO₂ injection can enhance geologic hydrogen generation while sequestering carbon, identifying optimal conditions for the process.

Mechanism and Key Findings

Natural hydrogen is produced when water reacts with iron-bearing minerals like olivine in ultramafic rocks through serpentinisation, a process that requires sufficient pore space for water flow. These same rocks can react with CO₂, turning it into solid carbonate minerals and permanently storing it underground, while simultaneously speeding up hydrogen production under certain conditions. The researchers found that CO₂ can increase natural hydrogen generation by up to 31% at depths where temperatures are below about 150°C (roughly 5,000 meters underground, depending on local geology).

The CO₂ dissolves in underground water, forming a weak acid that accelerates the dissolution of iron-bearing minerals, releasing more iron to react with water and produce hydrogen. At 123°C, the system with CO₂ generated 483 moles of hydrogen per cubic meter of olivine-rich rock within 3.5 years—31% higher than the 368 moles produced without CO₂. Simultaneously, 347 moles of CO₂ were mineralized into carbonate minerals (260 moles in magnesite and 87 moles in siderite).

Trade-Offs and Limitations

However, the reactions only proceeded for 3.5 years before pore space became clogged, which is 5.9 years shorter than the base case without CO₂. This faster clogging is not primarily caused by CO₂ turning into carbonate minerals. Instead, the increased acidity dissolves olivine faster, raising dissolved silica levels and favoring the formation of greenalite—a bulkier iron-silicate mineral that fills pores more quickly than cronstedtite (the mineral formed without CO₂). This leads to a quicker cessation of hydrogen-producing reactions.

The study suggests that magnesium-rich rocks may help mitigate this issue, as magnesium can react with dissolved silica to form magnesium-silicate minerals, removing silica from the fluid and reducing greenalite formation.

Implications and Future Research

The authors conclude that CO₂ mineralization could significantly reduce the amount of reactive formation needed to achieve net-zero emissions while contributing to CO₂ capture demand. They describe their results as revealing "specific coupled pathways" to meet net-zero goals and setting the stage for a potential "one-stop" solution for sustainable energy generation and CO₂ sequestration. However, they caution that these conclusions are based on simulations using only olivine, and further investigations are needed to fully understand the complexities of the reaction systems and realize the complete potential of this coupled scheme over space and time.